1. Field of the Invention
The present invention relates to a forming method of a semiconductor thin film, a forming device thereof, a forming method of a photovoltaic element, and a forming device thereof.
Specifically, when manufacturing a photovoltaic element comprising of at least two or more sets of laminated nip junctions, for example, a photovoltaic element comprising of a laminated type of cells having a double or triple cell structure, a semiconductor thin film with a reduced series resistance and a superior photo deterioration property can be obtained. Therefore, the invention relates to a method of forming a photovoltaic element superior in property uniformity and having fewer defects.
Also, the invention relates to a method and a device for forming a non-single crystal semiconductor thin film having a high quality and superior uniformity over a wide area, a high reproducibility and fewer defects, on a continuously moving belt-like member.
Further, the invention relates particularly to a forming device of a semiconductor thin film and a photovoltaic element which can maintain a high film-forming rate, while preventing particles (hereinafter referred to also as powder) generated in plasma from adhering to a substrate surface.
Especially, the forming method and device for forming the semiconductor thin film and the photovoltaic element according to the invention is preferable as a method and device using a roll to roll system for manufacturing in mass photovoltaic elements such as solar batteries or the like.
2. Related Background Art
As a conventional method of preparing a photovoltaic element using an amorphous silicon film (hereinafter referred to as the a-Si film) or the like, in general, a plasma CVD method is widely used and industrialized.
In the plasma CVD method, to form a high-quality i-type amorphous semiconductor layer or an n-type semiconductor layer, in a material gas of silane (SiH.sub.4) or the like, CH.sub.4, GeH.sub.4 or the like is appropriately mixed as a gas for adjusting a band gap, and the mixture gas is diluted (about once to 100 times) with hydrogen (H.sub.2). This process is often used. Alternatively, in another process, a high-frequency power is supplied low, generating a large number of long-life radicals to obtain a high-quality film through surface reaction.
However, to raise the deposition rate in the aforementioned plasma CVD method, the supply of a large quantity of required radicals and the promotion of surface reaction for relaxing a structure are necessary. For this purpose, increasing of a substrate temperature or other measures have been studied. The method, however, is disadvantageous in forming a P-I-N junction, and is not suitable for industrial applications.
As another measure for increasing the deposition rate, the density of high-frequency power is raised to decompose the material gas in large quantities. However, the radicals decomposed and generated with the high-density high-frequency power include a large quantity of SiH.sub.2 or other active radicals, and the structure cannot be sufficiently relaxed. As a result, a high-quality semiconductor film cannot be obtained. Further, the active radicals easily grow into clusters and, if further growing occurs, create fine particles (hereinafter referred to as powder). To solve the problem, for example, by applying a high-frequency power in a pulse form, the generation of powder is suppressed, while the powder generated by temporarily stopping the supply of plasma is discharged without being taken into a deposited film.
Also, to ensure that the photovoltaic element meets the electric power demand, the photovoltaic element is basically required to have a high photoelectric conversion efficiency, a superior property stability and a superior mass productivity at the same time.
To this end, when preparing the photovoltaic element using the a-Si film or the like, electric, optical, photo-conductive or mechanical properties, fatigue properties against repeated use, operation environmental properties or the like need to be improved. Further, together with increasing area and uniformizing film thickness and quality, mass production by means of high-rate film forming while attaining a high reproducibility must be targeted.
The semiconductor layer as an important constituting element of the photovoltaic element is provided with a so-called pn junction, pin junction or another semiconductor junction. When the a-Si or another thin-film semiconductor is used, the material gas including phosphine (PH.sub.3), diborane (B.sub.2 H.sub.6) or another dopant element is mixed with the main material gas of silane or another, and is decomposed by glow discharge, thereby obtaining semiconductor films having a desired conductive type. It is known that by successively laminating the semiconductor films on a desired substrate to form layers, the aforementioned semiconductor junction can be easily achieved. Further, it is proposed as a method of preparing an a-Si system photovoltaic element that an independent film-forming chamber is provided for preparing each semiconductor layer, i.e., each semiconductor layer is in each film-forming chamber.
For example, the specification of U.S. Pat. No. 4,400,409 discloses a continuous plasma CVD device using a roll to roll system. It is described in the specification that according to the device, a plurality of glow discharge regions are provided, and a flexible substrate having a sufficiently long desired width is arranged along a passageway through which the substrate successively passes the glow discharge regions. A desired conductive type of semiconductor layer is being deposited in each glow discharge region, while the substrate is continuously conveyed along its longitudinal direction, to continuously prepare the elements having a semiconductor junction. In the specification, a gas gate is used for preventing the dopant gas for use in preparing each semiconductor layer from diffusing or mixing into another glow discharge region. Specifically, the glow discharge regions are separated from one another via a slit-like separating path, and the separating path is provided with a means for forming a flow of Ar, H.sub.2 or another sweeping gas.
As a recently noted method for obtaining a high-quality film, there is a research report that in a capacitive coupling type plasma CVD method, a high-quality film is formed by using a self bias generated between a cathode electrode and an anode electrode and ion kinds. However, the research report has the following two problems.
(1) In an inner structure of a conventional typical discharge container, the area of the entire grounded anode electrode including a substrate is usually much larger than the area of a cathode electrode. Most of the supplied high-frequency power is consumed in the vicinity of the cathode electrode. As a result, the material gas is actively excited, decomposed and reacted only in a restricted portion, i.e., in the vicinity of the cathode electrode. The thin film forming rate becomes large only at the side where high-frequency power is supplied, i.e., in the vicinity of the cathode electrode. Even if a large quantity of high-frequency power is projected, the high-frequency power is not sufficiently supplied toward the anode electrode including the substrate. Therefore, it is indeed difficult to obtain a high-quality amorphous semiconductor thin film at a high depositing rate as desired.
(2) In the inner structure of the conventional typical discharge container, i.e., in the discharge container in which the area of the entire grounded anode electrode is much larger than the area of the cathode electrode, a positive bias is applied to the cathode electrode using a DC source. In the system, however, as a result of using the DC source as the secondary means, direct current flows in the plasma discharge. Therefore, when the direct current voltage bias is increased, spark or another unusual discharge is caused. It is very difficult to maintain a stable discharge by suppressing the unusual discharge. Therefore, it is unclear whether or not the application of direct current voltage to plasma discharge is effective. Because direct current voltage is not separated from direct current in the system. That is to say, a means for effectively applying only a direct current voltage to plasma discharge has been demanded.
Even when the technique stated in the aforementioned research report is used, powder is still generated in a film-forming space during high rate depositing, and is taken into the semiconductor thin film being deposited. The problem that the quality of the semiconductor thin film is adversely affected has not been solved.
In order to prepare a higher quality semiconductor thin film, it is indispensable to develop a technique for preventing the powder from being generated and taken into the semiconductor thin film being deposited.
Especially, the film thickness of the p-type semiconductor layer or the n-type semiconductor film forming the photovoltaic element is usually set as thin as several hundreds of angstrom in view of element properties. Therefore, when forming the photovoltaic element, especially the laminated layer type photovoltaic element, it is essential to prevent the powder from being generated and taken into the film being deposited. Not only the properties but the yield of the element are greatly influenced by the uniformity of layer thickness, the adherence of film, the doping efficiency of dopant, the uniformity of properties and the reproducibility.
Therefore, to obtain an a-Si thin film or another semiconductor thin film uniform in space and time and superior in reproducibility, a forming method and device are demanded in which the stability over a long period of time, reproducibility and uniformity of the electric discharge are enhanced. Further, when the throughput from the device is improved to save cost, a forming method and device are demanded which can increase the depositing rate while maintaining the quality of the semiconductor thin film.
Further, when forming a laminated type cell having a double or a triple cell structure, the conditions of preparing the aforementioned high-quality p-type semiconductor layer and further the n-type semiconductor layer laminated on the p-type semiconductor layer need to be optimized.
Especially, in the case of the preparing conditions with the aforementioned high depositing rate, the development of a measure for preventing the generation of powder and forming a high-quality amorphous semiconductor thin film has been demanded.
On the other hand, in a heretofore known method of continuously forming on a substrate a semiconductor deposit film for use in a photovoltaic element or the like, individual film-forming chambers are provided for preparing individual semiconductor layers, the film-forming chambers are connected via a gate valve in a road lock system, and the substrate is successively moved through the film-forming chambers to form each semiconductor layer. According to the method, a longitudinal belt-like member is used as the substrate. By continuously conveying the substrate in its longitudinal direction, while required conductive type semiconductor layers are deposited and formed in a plurality of glow discharge regions, the element having the semiconductor junction can be continuously formed.
However, the formation of a semiconductor film on a strip substrate extending several hundreds of meters requires several hours of film-forming time. Further, the semiconductor layer needs to be formed by maintaining and controlling the condition of electric discharge uniform and superior in reproducibility. A method is demanded in which a semiconductor deposit film of higher and uniform quality is continuously formed along the entire length of the longitudinal strip substrate from its commencing end to its terminal end with good yield.
For the i-type semiconductor layer for the photovoltaic element, for example, when an amorphous silicon or another thin film semiconductor is used, a main material gas of SiH.sub.4 (silane) or the like is mixed with H.sub.2 (hydrogen) and decomposed by glow discharge, to form the i-type semiconductor film can be formed. Especially, it is known the film quality of the i-type semiconductor layer determines the properties of the photovoltaic element. It is heretofore general that a high-quality i-type semiconductor layer is formed at a low depositing rate. On the other hand, to manufacture photovoltaic elements in large quantities at low cost, the enhancement of throughput from the forming device is demanded. It is clear to increase the throughput, the deposition rate of the semiconductor layer, especially the i-type semiconductor layers should be increased.
In the conventional technique, however, as the deposition rate is increased, the film quality of the i-type semiconductor layer is disadvantageously and remarkably decreased. A means for solving this problem is demanded.
In the inner structure of the conventional typical discharge container, the area of the entire grounded anode electrode including the substrate is usually much larger than the area of the cathode electrode to which power is applied. Most of the supplied high-frequency power is consumed in the vicinity of the cathode electrode. As a result, the material gas is actively excited, decomposed and reacted only in a restricted portion, i.e., in the vicinity of the cathode electrode. The thin film forming rate becomes large only at the side where high-frequency power is supplied, i.e., in the vicinity of the cathode electrode. Even if a large quantity of high-frequency power is supplied, the high-frequency power is not sufficiently supplied toward the side of the anode electrode or substrate. Therefore, it has been difficult to form an i-type semiconductor thin film on the substrate at a high deposition rate as desired. Further, it has been difficult to compatibly obtain an i-type semiconductor thin film of high quality and properties. Also, it is remarkably disadvantageous, in view of material cost, that more material gas is consumed in the vicinity of the cathode electrode, rather than on the substrate of a belt-like member or the like on which material gas must be deposited.
Also, in the inner structure of the conventional typical discharge container, i.e., in the discharge container in which the area of the entire grounded anode electrode including the substrate is much larger than the area of the cathode electrode, a positive bias is applied to the cathode electrode using a DC source. In the system, however, as a result of using the DC source as the secondary means, direct current flows in the plasma discharge. Therefore, when the direct current voltage bias is increased, spark or another unusual discharge is caused. It has been very difficult to maintain a stable discharge by suppressing the unusual discharge. Further, it has been unclear whether or not the application of direct current voltage to plasma discharge is effective. Direct current voltage is not separated from direct current in the system. That is to say, a means for effectively applying only a direct current voltage to plasma discharge has been demanded.
Therefore, to obtain a high quality i-type semiconductor layer uniform in space and time and superior in reproducibility, a forming device is demanded in which the stability over a long period of time, reproducibility and uniformity of the electric discharge are enhanced. Further, when the throughput from the device is improved to save cost, or when the efficiency of the material gas is enhanced, a device is demanded which can increase the deposition rate while maintaining the quality of semiconductor thin film. When a laminated type photovoltaic element is formed, a forming device is demanded which can continuously prepare an i-type semiconductor layer with higher reproducibility, uniformity and quality.
An object of the present invention is to solve various problems with the aforementioned conventional photovoltaic element forming device and provide a forming method and device of forming a non-single crystal semiconductor thin film, so that photovoltaic elements having a high photoelectric conversion efficiency, a high quality, a superior uniformity, a high reproducibility and fewer defects are produced in large quantities over a large area on continuously moving belt-like members.
Another object of the invention is to provide a semiconductor thin film forming device and a photovoltaic element which can prevent powder generated in plasma from adhering to a substrate surface while maintaining a high film-forming rate. By using the forming device, the quantity of powder taken into the film being deposited is decreased. Therefore, another object of the invention is to form an amorphous semiconductor thin film superior in photo deterioration properties and form a large quantity of photovoltaic elements having superior uniformity in properties and fewer defects, by using the aforementioned forming device.
A further object of the invention is to provide a method of forming a photovoltaic element superior in uniformity of properties and having fewer defects. According to the present invention, when manufacturing a photovoltaic element comprising at least two or more laminated nip junction layers, for example, a photovoltaic element comprising a laminated type of cells having a double or triple cell structure, a semiconductor thin film with a reduced series resistance and a superior photo deterioration property can be obtained. The forming method according to the invention is preferable as a method of manufacturing in mass photovoltaic elements such as solar batteries or the like using a roll to roll system.